In general, this invention relates to implantable pulse generators used to deliver stimulation to pace a patient's heart and particularly to those which detect atrial tachycardia and switch to either an antitachy pacing mode or a fallback mode on discovering indications of atrial tachycardia or atrial tachyarrhythmias.
In response to such indications, a pacemaker may perform in a number of different ways and it is not the purpose of this invention to detail such responses. Rather this invention is designed to prevent or reduce false responses. The usual response to tachy detection of current generation pacing devices is mode switching.
One method of responding to indications of tachycardia generally ignores the high intrinsic atrial rates and switches to ventricular pacing as described in U.S. Pat. No. 5,144,949 (Olson). This is a type of mode switching. Other systems use alternative operations in antitachy mode such as that described in U.S. Pat. No. 4,587,970 (Holley et at.) which uses reversion pacing to try to disrupt and discontinue the tachycardia. A fairly detailed background on pacemaker technology for mode switching is found in U.S. Pat. No. 4,562,841 (Brockway et at.).
Mode switching has had various definations and purposes in the pacemaker or pacing and pulse generator art. Examples, besides those mentioned above, include U.S. Pat. Nos. 5,284,491 (Sutton et at.), U.S. Pat. No. 4,856,523 (Sholder et al.), and U.S. Pat. No. 4,363,325 (Roline et at.). These use a sensor rate or some long term or changing atrial rate to determine when mode switching is to be done for the particular problems that those patents address. In general they are inventions responsive to problems developed because of or during rate adaptive pacing. Mode switching can best be described as where the pacemaker reverts to a mode that does not track (i.e., does not pace the ventricle in synchronization to) the atrial rate. Another way to say this is that in a mode switched condition pacing does not synchronize ventricular pacing to intrinsic atrial activity. Methods and features of mode switching are described in the Nov. 19, 1994 (Vol. 17, Part II) issue of PACE magazine in the article titled "A New Automode Switch Algorithm for Supraventricular Tachycardias" by Levine et al on pp. 1895-9. (Additional articles on automatic mode switching devices were also published in the same issue of PACE at pages 1900 (Den Dulk), 1908 (Ovsyshcher) and 1913 (Proverder).
Rate adaptive pacers which follow a patient's physiologic demand have been available for some time. A recent example is illustrated in U.S. Pat. No. 5,271,395 (Wahlstrand et at.). U.S. Pat. No. 4,856,524 (Baker, Jr.) uses an AV interval timer instead of an activity sensor (as in U.S. Pat. No. 5,052,388 to Sivula et at.) or minute ventilation (as in U.S. Pat. No. 5,271,395 Wahlstrand) to determine the appropriate pacing rate.
When mode switching is used however, certain timing sequences have been known to cause false positive indicators. That is the problem specifically addressed by this invention.
Pacemaker technology has been around for some 30 years. The technology for implanting such hermetically sealed electrical pulse generators (usually with batteries for power) responsive to a patient's pacing needs are well known in many aspects and those will not be described with particularity here. Instead, the reader should refer to descriptions available in the art cited in this application and other available literature.
In responding to this problem of false positive tachy indicators, the implantable pulse generator had been pacing at or near the programmed lower rate before, during, and after the erroneous indicator caused a pacemaker response. Commonly that response was referred to as a mode switch episode. These episode durations were generally short (less than one minute) but could be much longer. The AP-V-AR and AP-AR-V timing sequences (where the AP-AR interval is short relative to the size of the long interval AR-AP) sustained over multiple pacing cycles were generally felt responsible for false positive tachy detection and thus causing these inappropriate mode switch episodes. (Definitions: AP means atrial pace, V means ventricular event, AR means atrial refractory sense). Because a pacemaker senses these events through intracardiac electrical lead(s), it is mainly thought that these patterns can be the result of far field R-wave sensing or possibly from sensing retrograde P-waves. Test results using marker channel information (as described in U.S. Pat. No. 4,374,382 issued to Markowitz et al.) are available which show such occurrences as we have been describing. In general, a marker channel diagram is one indication of pacemaker interpretation of sensed cardiac or similar seeming noise events.
Our algorithms employ a timing variable which for convenience may be called the mean atrial interval (or MAI or Average Atrial Interval AAI) and represents what the pacing device or "pacemaker" considers the true atrial interval. Our AAI is also designed to converge on the shortest A-A interval (when long-short interval patterns are occurring). In certain situations this AAI/MAI algorithm can converge on the short AP-AR interval (described above) and thus identify such sequences as atrial tachycardia even in the absence of actual atrial tachycardia. This invention's algorithms can apply to any pacemaker that has false detection of tachycardia or atrial flutter, but one preferred embodiment is found in its application to the MAI in the Thera (TM) Medtronic pacemaker.
Other adjustments can also be made in the modem pacemaker which are described in reference to alternate preferred embodiments herein.